Passive stereo to mono solutions

Krakit

Krakit

Rzzzzz!
I will try to nutshell this as best as I can. I have a stereo output that I need to send into a mono input. I am looking to sum the stereo channels into the mono device without losing anything to phase cancelation and without damaging the output of the stereo device.

I have come across DIY instructions for adding a pair of resistors to a stereo 3.5mm output into a quarter inch mono input cable to supposedly achieve this effect. I'm not an expert in electronics but I don't see how adding resistance to the signal in both channels will prevent phase cancelation. (I can see how it will change the impedance to prevent the shorting damage from combining the two signals, but I need to have both).

Anyone know how to accomplish what I am trying to do? Please keep the solution passive if at all possible. I don't mind doing a little ham handed soldering to get er done.
 
Dave Matthews said:
Like this?
Click to expand...
Not quite I don't think.
That is a mono to stereo adapter, and will not sort any stereo to mono phase cancelling.

Personally, I would stick a resistor (say 100 ohms quarter watt) on each stereo source, then connect the other ends together
for the mono output. See how it sounds.
 
Slouching Raymond said:
Not quite I don't think.
That is a mono to stereo adapter, and will not sort any stereo to mono phase cancelling.

Personally, I would stick a resistor (say 100 ohms quarter watt) on each stereo source, then connect the other ends together
for the mono output. See how it sounds.
Click to expand...
Those are similar instructions to what I have found online so far, but I don't see how this deals with the phase cancelation issues. Can you explain it to me?
 
The two resistors just averages the two signals.
You're not going to fix phase cancelling with a passive circuit.
I know that you can remove vocals from a track, by subtracting one stereo source from the other,
because vocals are usually central, with an equal signal on left and right. Everything else is usually lop-sided,
so would not get cancelled.
But by using the 2 resistors to average the left and right signals, we are not subtracting (cancelling), but merely averaging them.
I think it would be ok.
 
Phase cancellation shouldn't be an issue with any normal stereo signal. The two signals will be mixed and will mostly add together rather than cancel. If someone has created a mix with too much that cancels when mixed to mono then it is simply a bad mix in my opinion (and I regularly query mixes that I receive for mastering that aren't mono compatible).
 
Hmmm, well full disclosure the sound source will be the left and right outputs (combined by headphones jack) from a keyboard. Not, as you are probably imagining a cell phone.
 
Krakit said:
Hmmm, well full disclosure the sound source will be the left and right outputs (combined by headphones jack) from a keyboard. Not, as you are probably imagining a cell phone.
Click to expand...
Since you are using a headphone output from a keyboard that makes it easy. Just buy a cable that joins left and right together at the destination. Most headphone outputs have a physical series resistor in circuit of about 33 to 100 Ohms. Combined, each output will 'see' the series result of each output R, i.e 66 to 200 Ohms. The amps won't mind a bit.
There ARE some headphones outputs that are made to have a (daft IMO) near zero OP Z and these could be damaged if shorted together but I doubt any keyboards do such a thing. For a 'general purpose' stereo summing cable yes, you should have about 1k Ohm in each side.

There will be no "phase anomalies" in the output of a synth and even if there were they will be 'baked' in and there is FA you can do about them.

Dave.
 
ecc83 said:
Since you are using a headphone output from a keyboard that makes it easy. Just buy a cable that joins left and right together at the destination. Most headphone outputs have a physical series resistor in circuit of about 33 to 100 Ohms. Combined, each output will 'see' the series result of each output R, i.e 66 to 200 Ohms. The amps won't mind a bit.
There ARE some headphones outputs that are made to have a (daft IMO) near zero OP Z and these could be damaged if shorted together but I doubt any keyboards do such a thing. For a 'general purpose' stereo summing cable yes, you should have about 1k Ohm in each side.

There will be no "phase anomalies" in the output of a synth and even if there were they will be 'baked' in and there is FA you can do about them.

Dave.
Click to expand...
I followed only about half of what you are saying but it sounds like I can use any mono to stereo cable in reverse with safety and effectiveness. I have such cables on hand already if that is the case. (Is OP Z the teenage engineering sequencer synth? That's all I get from a google search on it.)
 
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Sorry Dave ! "OP Z" is tekky speak for "Output impedance" because "Z" is the standard electronics character for "impedance*.
Worth mentioning I think that although the term 'impedance' is bandied about a great deal on audio forums, in practice the property we are referring to is almost always a nearly pure 'resistance'. Passive loudspeakers and to a lesser extent microphones have an impedance but the ins and outs of gear are just resistances and in the case of a headphone output an actual physical resistor.

It is usually bad practice to simply short a left and right output. With modern kit there is little chance of damage but it can result in some distortion. Headphone outs are generally an exception.

*IIR my tech college days, impedance" was always lower case 'z' but I always type it upper case to differentiate it from other letters in a text. Like death and taxes Dave I am always with you here so any more clarification needed, just shout.

Dave.
 
If your keyboard output has a phase problem, that's inherent to the way the patches were created. No special way of summing them is likely to avoid it. Sometimes it's better to take one side of the stereo, like the right side of a piano (which I might do in a rock band situation anyway). But obviously you can't do that if there's something that needs to be there, like a ping-pong delay.
 
Loads of keyboards and synths layer their sounds with reverbs created from delays and choruses, and when you have them left and right connected, they sound great. Summing them usually makes them thinner and weedy because the delays and choruses just partially cancel out. Dave's like on just joining them up is my experience. I've been shorting them together since 1970 something and never ever caused any issues at all - given that many musicians are totally untechnical - the designers realise a stereo ¼" doubt adaptor could easily be used to mono a left and right - so they made sure that shorts, open circuits and silly things were bombproof.

Those two resistors are simple so they act as a buffer. The short is not a short. Phase cancellation is just what happens, and you either live with it, or find an extra input and pan wide. After all two channels from a typical keyboard sound weedy when the pan knobs for both are in the same position.
 
rob aylestone said:
Loads of keyboards and synths layer their sounds with reverbs created from delays and choruses, and when you have them left and right connected, they sound great. Summing them usually makes them thinner and weedy because the delays and choruses just partially cancel out. Dave's like on just joining them up is my experience. I've been shorting them together since 1970 something and never ever caused any issues at all - given that many musicians are totally untechnical - the designers realise a stereo ¼" doubt adaptor could easily be used to mono a left and right - so they made sure that shorts, open circuits and silly things were bombproof.

Those two resistors are simple so they act as a buffer. The short is not a short. Phase cancellation is just what happens, and you either live with it, or find an extra input and pan wide. After all two channels from a typical keyboard sound weedy when the pan knobs for both are in the same position.
Click to expand...
I think I will cross my fingers and give this a shot. It will have to wait a bit to try out because....reasons.
 
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